The present invention relates to an oscillation circuit.
Conventionally, an oscillation circuit as shown in FIG. 1 has been proposed as an oscillation circuit which can be produced on a semi-conductor substrate and operated by low power-supply voltage. This oscillation circuit is of the well-known ring type produced by complementary MOS semi-conductors, and comprises first through fourth unit circuits 6.sub.1 -6.sub.4 and a control circuit 7. The first through fourth unit circuits 6.sub.1 -6.sub.4 and the control circuit 7 are cascade-connected to each other. The control circuit 7 is provided between the second unit circuit 6.sub.2 and the third unit circuit 6.sub.3. As shown in FIG. 2, the first unit circuit 6.sub.1 comprises first and second inverter circuits 61.sub.1, 61.sub.2 and a delay circuit. The first and second inverter circuits 61.sub.1, 61.sub.2 are cascade-connected to each other. The delay circuit consists of a resistor R51 and a capacitor C51. The resistor R51 is provided between the first inverter circuit 61.sub.1 and the second inverter circuit 61.sub.2. The capacitor C51 is provided between the connection point of the resistor R51 with the second inverter circuit 61.sub.2 and the earth. The first inverter circuit 61.sub.1 functions as a switch for charging and discharging the capacitor C51. The second inverter circuit 61.sub.2 functions as a waveform shaper of an output signal of the delay circuit. The structures of the second through fourth unit circuits 6.sub.2 -6.sub.4 are similar to that of the first unit circuit 6.sub.1. The control circuit 7 has the well-known structure of 2NAND. When a control signal CNT inputted as one of two input signals is of high level, the control circuit 7 outputs a signal by inverting the polarity of the other input signal which is an output signal of the second unit circuit 6.sub.2. An output signal of the fourth unit circuit 6.sub.4 is fed back to the first unit circuit 6.sub.1.
The capacitor C51 is a MOS capacitor. The capacitor C51 with capacitance in the order of several pF can be produced on a semi-conductor substrate when viewed in terms of its area. The resistor 51 can be produced on the semi-conductor substrate according to its resistance by means of one of three methods described below.
(1) The method using the diffusion layers of P+ type and N+ type for forming the source and drain of a MOS transistor, when the resistance per unit area is of the order of 50 through 100 .OMEGA..
(2) The method using poly-crystal silicon for forming the gate electrode of a MOS transistor, when the resistance per unit area is of the order of 20 through 40 .OMEGA..
(3) The method using the ion implantation process for producing the resistor R51 should be added, when the resistance per unit area is of the order of 1 through 4 k.OMEGA..
The temperature characteristics of the resistor R51 produced on the semi-conductor substrate according to the above three methods differs for each method. The temperature coefficient of the resistor R51 generally varies according to method used, with the method using poly-crystal silicon giving the lowest temperature coefficient, and the method involving the newly added ion implantation process providing the highest temperature coefficient. One of the above three methods is therefore selected according to the area on the substrate allocated for producing the resistor R51 and the characteristics to be required of the resistor R51. By means of one of the above three methods, it is possible to produce a resistor R51 with resistance of the order of several tens of k.OMEGA. on the semi-conductor substrate. The gate width of the first inverter circuit 61.sub.1 is determined so that the output resistance may become sufficiently smaller than the resistance of the resistor R51.
The oscillation frequency f of the oscillation circuit shown in FIG. 1 is given by the following equation. EQU 1/f=4.times..tau..sub.r +4.times..tau..sub.f +.tau..sub.0 ( 1)
In equation (1), .tau..sub.r denotes the delay time of the delay circuit generated through a charging operation, .tau..sub.f the delay time of the delay circuit generated through a discharging operation, and .tau..sub.0 the delay time of the control circuit 7. Usually, .tau..sub.0 is sufficiently smaller than .tau..sub.r and .tau..sub.f. The oscillation circuit shown in FIG. 1 can oscillate up to a frequency f of the order of several MHz. However, the duty factor of the output signal is determined by the ratio of .tau..sub.r to .tau..sub.f. Since it is difficult to obtain the relation .tau..sub.r =.tau..sub.f because of the dispersion to be generated in the production process, the duty factor of the output signal generally deviates from 50%.
As a method for preventing the duty factor of the output signal from deviating from 50%, there is the known method for making each unit circuit output a signal with the polarity opposite to that of the input signal. An example of the structure of the unit circuit of this method is shown in FIG. 3. The difference of a unit circuit 6a shown in FIG. 3 from the unit circuit 6.sub.1 shown in FIG. 2 is that a third inverter circuit 63.sub.1 is added to the former for inverting the polarity of the output signal of the second inverter circuit 62.sub.1. In the unit circuit 6a, since the duty factor is determined by the ratio (.tau..sub.r +.tau..sub.f) (.tau..sub.r +.tau..sub.f), it is possible to reduce the dispersion of the duty factor. However, the unit circuit 6a has a relatively large dependence on the power-supply voltage.
As an example of unit circuit for solving the above problem regarding power-supply voltage, it is conceivable to use a unit circuit 6b as shown in FIG. 4, which includes a Schmidt circuit 64.sub.1 having input threshold with hysteresis characteristic in place of the second inverter circuit 62.sub.1 of the unit circuit 6.sub.1 shown in FIG. 2. By constructing the oscillation circuit by using the unit circuits 6b instead of the first through fourth unit circuits 6.sub.1 -6.sub.4, the dependence of the oscillation frequency on power-supply voltage is obtained by means of the circuit simulator. An example of the result is shown in FIG. 5. With this result, it has been confirmed that in the oscillation circuit composed by using the unit circuit 6b, the dependence of the oscillation frequency on power-supply voltage is scarcely detected even with power-supply voltage of not less than 3 volts, and thus stable oscillation is obtained when the power-supply voltage is in the low range of the order of three volts. However, it is also found that even in this oscillation circuit the dependence of the oscillation frequency on the power-supply voltage rapidly increases when the power-supply voltage becomes less than 3 volts. Therefore, when this oscillation circuit is used for a portable unit in which a power-supply voltage of about 2 volts is required for the semi-conductor integrated circuit, stable oscillation frequency cannot be obtained.